adaptability, provided at the settlement site (time, place, amount of local signals). Sewell Wright was prescient, we think, when he noted in 1931, “The older writers on evolution were often staggered by the seeming necessity of accounting for the evolution of fine details … for example, the fine structure of all of the bones … structure is never inherited as such, but merely types of adaptive cell behavior which lead to particular types of structure under particular conditions.”
Although we concur that externally directed phenotypic plasticities are a rich source of variations for regulatory stabilization, we add to it the richer source of internally directed cellular developmental adaptations. The latter class would not be evoked by the environment and then stabilized, but stabilized directly by regulatory change driven by genetic variation.
Thus far we have discussed how conserved core processes facilitate regulatory change, but we should also discuss how various regulatory processes, evolved in pre-Cambrian animals, have facilitated the use of core processes in different combinations, amounts, and states, while decreasing their chances of interference (pleiotropy). Spatial compartmentation of transcriptional regulation and cell–cell signaling is one of these.
In bilateral metazoa, the body of the mid-stage embryo, sometimes called the phylotypic stage of development, becomes divided into a regulatory grid or map of small compartments, each uniquely defined by its expression of one or a few selector genes encoding transcription factors or signaling molecules. The insect embryo at this stage contains ≈100 contiguous compartments, and the vertebrate embryo contains perhaps 200. The map is highly conserved within a phylum, and the stage is called phylotypic because embryos of all classes of the phylum then look most similar. Thereafter, selector genes of a compartment specify the anatomy and physiology to be developed within it; they “select” other genes, some encoding regulators and some encoding core process components, to be expressed or repressed in their compartment, thereby combining and customizing core processes for local usage. Different combinations, amounts, and states of core processes can be engaged in parallel in numerous regions of the embryo (Schlosser and Wagner, 2004; Carroll, 2005a). Conflicting processes such as cell death and proliferation can be run separately without interference.
One example of compartmentation is found in developing vertebrae, all of which contain bone-forming cells. In thoracic vertebrae they also form ribs, whereas in the cervical vertebrae they do not. Despite their equivalence as bone-forming cells, they differ, as shown by transplan-